1. Field of the Invention
The present invention relates to a dye-sensitized or dye doped solar cell in which light energy is directly converted into electric energy, and a method for manufacturing such dye-sensitized or dye doped solar cells.
2. Background Art
When a photovoltaic material is irradiated with light, electrons restricted to an atom in the photovoltaic material are released by light energy to move freely, which generates free electrons and holes. The free electrons and the holes are efficiently separated so that electric energy is continuously extracted. That is, the photovoltaic material is capable of converting light energy to electric energy. Such photovoltaic material has been utilized as a solar cell and the like.
The common solar cell is produced by first forming an electrode on a support such as a glass plate coated with a transparent conductive film, subsequently forming a semiconductor film having a photosensitizer adsorbed thereon on a surface of the electrode, thereafter providing a counter electrode comprising a support such as a glass plate coated with another transparent conductive film, sealing an electrolyte between the counter electrode and the semiconductor film, and finally sealing the side faces with a resin or the like.
When the above semiconductor film is irradiated with sunlight, the photosensitizer adsorbed on the semiconductor absorbs visible-region rays to thereby excite itself. Electrons generated by this excitation move to the semiconductor, next to the transparent conductive glass electrode, and further to the counter electrode across a lead connecting the two electrodes to each other. The electrons having reached the counter electrode reduce the oxidation-reduction system in the electrolyte. On the other hand, the photosensitizer having caused electrons to move to the semiconductor is in oxidized form. This oxidized form is reduced by the oxidation-reduction system of the electrolyte to thereby return to the original form. In this manner, electrons continuously flow. Therefore, functioning as the solar cell can be realized.
The electrolyte to be sealed between the electrodes is dissolved in a solvent, selected according to the type of the electrolyte, to thereby obtain an electrolytic solution. The electrolytic solution is sealed in a cavity created by sealing the sides of the photovoltaic cell with, for example, a resin.
The above solvent can be selected from among, for example, water, alcohols, oligoesters, carbonates such as propione carbonate, phosphoric acid esters, dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, N-vinylpyrrolidone, sulfur compounds such as sulfolane 66, ethylene carbonate and acetonitrile.
One dye-sensitized solar cell that has widely drawn attention because of its higher photovoltaic efficiency comprises percolating networks of liquid electrolyte and dye-coated sintered titanium dioxide and was developed by Dr. Michael Grätzel and coworkers at the Swiss Federal Institute of Technology. The solar cell is comprised of an electrolyte solution layer sandwiched between a semiconductor electrode and a counter electrode. When the semiconductor electrode is irradiated with light, electrons are excited therein and transferred to the counter electrode through an electric circuit. The transferred electrons are re-transferred as ions to the semiconductor electrode through the electrolyte. This cycle is repeated to extract electric energy.
The Grätzel solar cell, as shown in FIG. 1, includes a transparent substrate 1 on one side of which a transparent conductive film 7a is formed, and a conductive substrate 5 on which a transparent conductive film 7b and a semiconductor electrode (dye-sensitized semiconductor electrode 4) carrying a sensitizing dye is formed. The two substrates are stacked one upon the other with an electrolyte 3 contained there between, and the assembly is sealed with resin 8. A porous titanium oxide film is provided on the surface of the conductive substrate and is coated with a sensitizing dye that can efficiently absorb solar light, such as ruthenium complex. The porous titanium oxide film is used as a dye-sensitized semiconductor electrode 4, whereby an electron excited by light is injected into the titanium oxide and a flow of electric current can be caused. This type of solar cells requires electrolyte for the exchange of electrons, and generally iodine electrolyte is used for this purpose.
In the conventional dye-sensitized solar cell shown in FIG. 1, the electrolyte is only sealed and contained by a thick coating of resin at the peripheral portion (near the cross section), which is then cured. However, the amount of electrolyte may vary due to different intervals between the two substrates from one solar cell to another, or even within each solar cell. As a result, there have been problems in terms of reproducibility and safety due to the leakage of the electrolyte that may be caused when the solar cell is inclined, for example.
When the electrolyte is used in the form of such an electrolytic solution, the solar cell may suffer from, during a long-term service, alteration of solvent molecules, decomposition of solvent molecules, vaporization of low-boiling-point solvent, leakage of electrolytic solution (solvent and/or electrolyte) from sealed parts, etc. with the result that the performance, such as photovoltaic transduction efficiency, thereof is deteriorated. That is, the use of the electrolyte in the form of an electrolytic solution has a drawback in that the long-term stability is poor.
Moreover, depending on the type of electrolyte used in the electrolytic solution, hygroscopicity is exhibited to thereby absorb water, and it may occur that the water causes the electrolyte and the photosensitizer to decompose to result in performance deterioration.
Furthermore, the surface irregularities of the porous titanium oxide film used as the dye-sensitized semiconductor electrode vary depending on the coating method used, the particle diameter, or the thickness of the film. Should a projecting portion of the film come into contact with the conductive film on the opposite substrate, the dye-sensitized semiconductor electrode will be electrically in contact with the conductive film, bypassing the electrolyte. This would prevent the sufficient exchange of electrons, and lead to decrease in efficiency and destabilization of performance of the solar cell.
Conventionally, fabrication of dye-sensitized solar cells requires a high temperature sintering process (> about 400° C.) to achieve sufficient interconnectivity between the nanoparticles and enhanced adhesion between the nanoparticles and a transparent substrate.
Although the photovoltaic cells of a Grätzel cell are fabricated from relatively inexpensive raw materials, the high temperature sintering technique used to make these cells limits the cell substrate to rigid transparent materials, such as glass, and consequently limits the manufacturing to batch processes and the applications to those tolerant of the rigid structure. Rigid substrates are fragile, heavy, inflexible, and costly to manufacture, and consequently, they do not lend themselves to conventional shelf pricing systems powered by photovoltaic devices.
The photovoltaic transduction efficiency of conventional solar cells is not always satisfactory, and there are limitations in the application thereof. Therefore, there is a constant demand for improved solar cells and further enhancement of light utilization ratios.
It is a further aspect of the present invention to provide a photovoltaic cell which is safer, eliminates troublesome components such as an electrolyte, has excellent long-term stability, ensures a high light utilization ratio, and exhibits high photovoltaic transduction efficiency.